Sailing adrift, setter leaves message (6)
I believe the answer is:
signal
'message' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for signal that I've seen before include "Gesture; notable" , "Message in the form of a gesture" , "beacon" , "Noteworthy" , "Non-verbal action with an encoded message" .)